This invention relates to modifiers for vinylidene chloride resins, and more particularly to such modifiers containing ethylene-vinyl acetate copolymers.
Vinylidene chloride resin composites are advantageous in that the composites have low permeability to moisture and gases. They are, hence, especially suitable a food wrapping material which preserves foods without adversely affecting their fragrance or taste and without damaging their quality. In order to improve extrusion condition of such composites, plasticizers, such as dioctyl phthalate, dibutyl sebacate, .alpha.-chloro-naphthalene, soybean oil epoxide, and the like have been used.
However, since these compounds are not very compatible with vinylidene chloride resin, they usually exude out from the composite. Thus, composites having such plasticizers are not desirable from the standpoint of sanitation especially when used in direct contact with food because the plasticizer may be transferred to the food.
The foregoing disadvantage can be removed by using polyester plasticizers (e.g. polymer of glycerine or glycols and dibasic acid such as adipic acid, azelaic acid, or sebacic acid), or elastomers instead of the plasticizers of low molecular weight mentioned above. However, polyester plasticizers also have disadvantages in that their film forming property is not always satisfactory and any film containing the polyester plasticizer becomes brittle and fragile at low temperatures. As for film containing elastomer as a plasticizer, it is possible to eliminate the disadvantage that the film becomes brittle and fragile at low temperatures, but still it is generally difficult with elastomers to obtain film composites having antishock property at low temperatures because homogeneous mixing to form transparent film is difficult due to the poor mutual compatibility between the elastomer and the vinylidene chloride resins.
Therefore, it is desirable to develop a modifier for vinylidene chloride resins, which does not exude out from the composite and gives sufficient strength at low temperatures to films made thereof. Also, it is desired that such modifier does not decrease the excellent transparency and low permeability against moisture and gases, which are general advantages of vinylidene chloride resin composites.
Several proposals have been made on solving the foregoing problems involving vinylidene chloride resin composites containing ethylene-vinyl acetate copolymers. Belgium patent 7211565 claims that ethylene-vinyl acetate copolymer is directly mixed with vinylidene chloride resins. Japanese patents 47-11663 and 11664 describe a three component mixture containing ethylene, vinyl acetate and vinyl chloride or alkyl esters of acrylic acid. There are descriptions on graft copolymer composites which are obtained by the polymerization of vinylidene chloride under the presence of (a) ethylene-vinyl acetate copolymer, such as in Japanese Pat. No. 45-5066. (b) a three component copolymer of ethylene, vinyl acetate and vinyl chloride, such as in Japanese Pat. No. 45-39103, and (c) an ethylene-vinyl acetate copolymer and chlorosulphonated polyethylene, such as in Japanese Pat. No. 45-39102. None of these proposals has been entirely satisfactory.